Polychlorobiphenyls (PCB) are synthetic chlorinated organic compounds derived from biphenyl. They are 209 congeners of PCBs, which differ in the number and position of chlorine atoms on the biphenyl backbone.
Polychlorobiphenyls (PCB) are environmental pollutants which causes great problems. PCBs are globally distributed compounds, characterized by their non reactive chemical structure and low water solubility. In general these compounds are considered non degradable (Kalmaz et al., 1979, Review. Ecol. Model 6:223-251). PCB-contaminated waste is a major component of many polluted environmentally hazardous sites, which require treatment or remediation.
PCBs were mainly produced and commercialized in North America, Europe and Japan during 60 years, from the beginning of the 1930s. The commercial mixtures of the PCBs such as Aroclor (Monsanto, United States), Phenoclor and Pyralene (Prodelec S.A., France), Clophen (Bayer AG, Germany) and Kanechlor (Kanegafuchi Chemical Industrial Co. Ltd., Japan) contain between about 60 and 80 different congeners. PCBs have been used in the industry as dielectrics fluids in transformers and capacitors, hydraulic and heat transfer fluids and fire retardants, among other applications. In addition, they could be in lubricants, paints, plastics and adhesives. The enormous industrial application of these compounds is due to their extraordinary physical and chemical properties, i.e., oily consistency, great stability, chemical inertia, heat resistance and high dielectric constant. Nevertheless, the chemical stability of PCBs also prevents its degradation in the environment. On the other hand, the high hydrofobicity of these compounds allows their solubility in organic solvents, oils and fats, which promotes their accumulation through the trophic chains. This phenomenon is known as biomagnification. PCBs have been detected worldwide, including non-industrialized zones located away from their industrial sources such as the poles. PCBs cause world-wide concern after the accidental contamination of rice oil in Japan and Taiwan by the end of 1968 and 1979, respectively. These events related PCBs to serious effects on human and animal health (Yao et al., 2002; Chen et al., 1992). Due to these events the international community becames aware of the necessity to regulate the production, use and elimination of the PCBs. Different types of cancer have been related to these compounds, especially liver and digestive system cancers. PCBs has been related also with reproductive disorders, development deficiencies, anomalities in the immunological system, endocrine system disorders, neurological damages and cutaneous injuries. In May, 2001 during the Convention of Stockholm, the problem of the persistent organic pollutants (POPs) was discussed. As a result a list of 12 POPs including the PCBs were classified as POPs for priority action. In 1976, in the United States as well as in Europe the first legal figures were established to limit and to eventually erradicate the commercialization and manipulation of PCBs. In Chile the use of PCBs in new electrical equipments such as transformers and converters is actually forbidden. Nevertheless, electrical equipment containing PCBs, which were produced before the date of prohibition, are still used. According to the project entitled “National Diagnosis of Persistent Organic Pollutants” promoted by the CONAMA (Spanish acronym of Environmental National Commission), there are huge amounts of PCBs stored and in use in Chile (at least 396,705 liters). The II and III regions have the main amounts of PCBs followed by the VIII and Metropolitan regions (Eula-Chile Center, 2001). Different PCB congeners have been detected in bivalves, fish muscle, bird eggs and muscle and sediments in different regions of the country (Focardi et al., 1996; Fuentealba, 1997; Mũnoz and Becker, 1999; Barra et al., 2004). Different methods are currently known to destroy PCBs. These methods are based on physical, chemical or physico-chemicals processes such as incineration, thermal desortion, chemical dechlorination, solvent extraction, ground washing and immobilization. Nevertheless, all these techniques involve expensive operation costs and most of them cannot be applied to contamination sites of wide water or soil extensions. The biological degradation by microorganisms represents an attractive alternative for the implementation of bioprocesses for the elimination of PCBs and the remediation of soil contaminated with these compounds.
Diverse microorganisms capable of degrading biphenyls and their chlorinated derivatives, both in aerobic and anaerobic conditions have been described. The aerobic bacterial degradation have been widely studied, which allowed the characterization of a complete catabolic route. Diagram 1 represents the aerobic process of biodegradation of PCBs.

The compound (chloro)biphenyl (1) is dioxygenated by a biphenyl-2,3-dioxygenase to form the product biphenyl-dihydrodiol (2). This compound is then dehydrogenated by a biphenyl-dihydrodiol dehydrogenase to form 2,3-dihydroxy-biphenyl (3). Subsequently, a new dioxygenation carried out by a 2,3-dihydroxybiphenyl-1,2-dioxygenase generate the compound 2-hydroxy-6-oxo-6-phenyl-hexa-2,4-dienoate (4). This compound is finally transformed by an hydrolase into 2-hydroxypenta-2,4-dienoate (5a) and (chloro)benzoate (5b). This step is the last reaction of the upper biphenyl degradation pathway. The lower catabolic pathway catalyzes the oxidation of 2-hydroxypenta-2,4-dienoate (5a) into intermediates of the metabolism to yield energy. As a first stage of the lower pathway, a hydratase produces the compound 4-hydroxy-2-oxovalerate (6). Then an aldolase produces the compounds acetaldehyde (7a) and piruvate (7b). The last step of the lower biphenyl pathway is the acetaldehyde CoA generation by an acetaldehyde dehydrogenase.
The bacterium Burkholderia xenovorans LB400, which was isolated about the middle of the 1980s by a team of the General Electric company from a garbage dump contaminated with chloroaromatics in the state of New York, U.S.A., is one of the most potent PCB-degrading microorganism. This bacterium is capable to attack a wide range of highly-chlorinated congeners including hexachlorinates ones due to the relaxed specificity of its catabolic enzymes. Nevertheless, like most of the PCB-degrading strains, which have been characterized until now, it is not capable to degrade chlorobenzoates (CBA). This leads to the accumulation of CBAs during the bacterial degradation of PCBs. This can lead to the potential generation of highly toxic compounds by the environmental microflora such as the protoanemonin antibiotic. This phenomenon considerably decreases the degradative process and constitutes a significant problem for designing bioremediation strategies for PCBs. In order to avoid this problem and to improve the PCB bioremediation, several studies have been carried out to obtain microorganisms and microbial consortia that are capable to degrade a wide range of PCBs as well as CBAs. Some of these efforts to obtain an optimal biological system for PCB-remediation are: U.S. Pat. No. 4,843,007, Bedard et al., Jun. 27, 1989; U.S. Pat. No. 4,843,009, Bopp, Jun. 27, 1989; U.S. Pat. No. 5,009,999, Bopp, Apr. 23, 1991; U.S. Pat. No. 5,968,360, Crowley et al., Oct. 19, 1999; U.S. Pat. No. 6,287,842, Dyadischev et al., Sep. 11, 2001; U.S. Pat. No. 3,148,501, Sanggoo and Flynn, Aug. 7, 2003. The following publications are also included: Mokross et al., 1990, FEMS Microbiol. Lett. 59:179-185; Havel and Reineke, 1991, FEMS Microbiol. Lett. 62:163-169; Adams et al., 1992, Appl. Environ. Microbiol., 58:647-654; Hofer et al., 1996, Molecular Biology of Pseudomonads; Potrawfke et al., 1998, Appl. Microbiol. Biotechnol. 50:440-446; Hrywna et al., 1999. Appl. Environ. Microbiol, 65:2163-2169; Klemba et al., 2000, Appl. Environ. Microbiol. 66:3255-3261; Rodrigues et al., 2001, Environ. Scien. Technol. 35:663-668; Sanggoo et al., 2001, Appl. Environ. Microbiol. 67:1953-1955.
The construction of recombinant bacteria with improved PCB-degrading capacities have been not very successful. This could be explained by the limited substrate range of the bacteria, and the poorly known genetic systems. A way to overcome these limitations is to use well-characterized systems with known catabolic capabilities.